1. Field of the Invention
This invention relates to methods for improving the grinding and polishing of glass, ceramics and semi-conductor materials.
2. Description of the Related Art
Referring to FIG. 2A, a blank 2 of fused silica is shown. The subsurface damage layer 6, which contains mainly fractures that have been covered by the polishing re-deposition layer 4, is a likely contributor to laser induced damage. This subsurface damage (SSD) may reduce the strength of the final polished part by providing initiating cracks that reduce fracture strength. SSD may provide sites for light-absorbing contaminants to reside. When these contaminants are at or near fracture surfaces, the atoms are more easily ionizable (by changing the chemical or electronic environment), which can cause larger cracks and fractures. SSD may also modulate locally the electromagnetic field.
Subsurface damage is eliminated or minimized in practice by using a controlled sequence of successively gentler grinding and polishing steps, making sure that each step removes enough material to eliminate damage produced by the previous step. Studies of the effect of subsurface damage on laser damage threshold have primarily been at longer wavelengths than 355 nm. The characteristics of laser induced damage are better understood at these longer wavelengths; therefore, grinding and polishing methods of the prior art have been successful. However, the characteristics of laser induced damage at shorter wavelengths is not well understood in the prior art.
The laser induced damage threshold of fused silica and calcium fluoride optics is an area of critical importance to the high energy fusion laser community and the multi-billion dollar semiconductor capital equipment market. In the lithography equipment for manufacturing of silicon chips, a UV light in the range of 340-360 nm is primarily used. However, manufacturers would like to use shorter wavelengths such as 193 nm and 248 nm. These UV wavelengths are becoming common in biomedical devices as well. All of these wavelengths are produced by a series of UV lasers, and imaged through fused silica and CaF optics, which are susceptible to laser induced damage. Unfortunately, present grinding and polishing techniques can not produce quality optics to be used with these short wavelengths.